Abstract
Hetero-junction organic photovoltaic (OPV) cells consisting of donor (D) and acceptor (A) layers have been regarded as next-generation PV cells, because of their fascinating advantages, such as lightweight, low fabrication cost, resource free, and flexibility, when compared to those of conventional PV cells based on silicon and semiconductor compounds. However, the power conversion efficiency (η) of the OPV cells has been still around 8%, though more than 10% efficiency has been required for their practical use. To fully optimize these OPV cells, it is necessary that the low mobility of carriers/excitons in the OPV cells and the open circuit voltage (V OC), of which origin has not been understood well, should be improved. In this review, we address an improvement of the mobility of carriers/excitons by controlling the crystal structure of a donor layer and address how to increase the V OC for zinc octaethylporphyrin [Zn(OEP)]/C60 hetero-junction OPV cells [ITO/Zn(OEP)/C60/Al]. It was found that crystallization of Zn(OEP) films increases the number of inter-molecular charge transfer (IMCT) excitons and enlarges the mobility of carriers and IMCT excitons, thus significantly improving the external quantum efficiency (EQE) under illumination of the photoabsorption band due to the IMCT excitons. Conversely, charge accumulation of photo-generated carriers in the vicinity of the donor/acceptor (D/A) interface was found to play a key role in determining the V OC for the OPV cells.
Highlights
Sou Ryuzaki graduated with his Ph.D from Department of Nuclear Engineering at Tokyo Institute of Technology in March 2010
Because it was confirmed that the surface morphology of each Zn(OEP) film used here remained almost unchanged by SEM [20], the C60/Zn(OEP) interfacial states and the contribution of excitons generated in the C60 film are the same as both the Organic photovoltaic (OPV) cells, allowing us to discuss the structural effects of the Zn(OEP) films on the difference in external quantum efficiency (EQE) between them
To investigate the electronic states of the Zn(OEP) and C60 films near the donor/ acceptor (D/A) interface under photoirradiation, the impedance spectra of the OPV cells were measured under dark and photoirradiation with wavelengths of 400, 500, and 590 nm, and subsequently the CÁV characteristics of each film near the D/A interface were examined under each condition
Summary
Jun Onoe graduated from the M.S. course of the department of Chemistry, Osaka University in 1987, and got a Doctor of Science from the Department of Physics, Kyoto University in 1997 After He worked for Panasonic (1987Á 1989) and the Institute of Physical and Chemical Research (RIKEN) (1990Á2002), he has been an Associate Professor of Research Laboratory for Nuclear Reactors and Department of Nuclear Engineering, Tokyo Institute of Technology since 2002. His current research subjects are [1] Exotic-Nanocarbon Materials, [2] Organic Solar Cells, and [3] Nuclear Materials. The ISC of the OPV cells is generated by the four processes (efficiencies) shown in Fig. 1c: 1)
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